Apparatus for controlling electrical precipitators



May 3, 1960 M. P. FOLEY 2,935,155

APPARATUS FOR CONTROLLING ELECTRICAL PRECIPITAToRs Filed July 9, 1954 ,2.F-c: J. g

JaL 1 BY 4l/5,9465 Canale5/v7- f/IJA) i; 41

Trae/Vey;

yUnited States Patent APPARATUS FOR CONTROLLING ELECTRICAL PRECIPITATGRSMichael P. Foley, Los Angeles, Calif., assigner, by mesheassignments, toJoy Manufacturing Company, a corporation of Pennsylvania ApplicationJury 9, 1954, seran No. 442,415

-4 claims. (ci. 18s- 7) p ciency.

VFor most eliicient operation in an electrical precipitator, thedischarge current between the electrodes should bel maintained as highas possible inasmuch as it is this discharge current that eiectivelychanges particles to be collected, and thus causes them to be attractedto the electrodes of the precipitator. The upper limit of the dischargecurrent is established by that Value of the inter-electrode voltage atwhich arc-over between the electrodes occurs. It is found that justprior to arc-over, a certain amount of sparking between theelectrodestakes place. This sparking is not serious except insofar as it isindicative of possible arc-overs occurring. Actual arcing between theelectrodes is serious in that it puts large loads on the rectier andother generating' components and particularly because it drops thevoltage between the electrodes to a value where particle collection isvery poor.

lIt is highly desirable accordingly, to operate a precipitator in such amanner that the average discharge current is |at an optimum value formaximum particle collection eiciency and yet, below the point at whicharcover will occur. Various attempts in this direction have been made.For example, it has been found that the discharge current is close tooptimum value when a certain sparking rate is achieved. Suitable voltageregulating apparatus responsive to a predetermined sparking rate for anyone precipitator may then be employed for ad-v justing the high voltageacross the precipitator electrodes to a value which will maintain thedischarge current at the particular value for attaining the givensparking rate. A weakness of this system is that other factors enterinto determining the optimum value of discharge current so thatitherelation between sparking rate and current is not always constant.Accordingly arc-overs can occur when very little sparking ispresent. Onthe other hand, these' other fators can result in a high rate ofsparking even` though the discharge ycurrent could be safely increasedwithout fear of arc-over. Probably, as many as twenty variable factorsaftectthe optimum value of the discharge current and as a result thedesign ofsuitable regulating apparatus; that regulates by reference toonly one ofthe factors involved is well nigh impossible.

The present invention has as its primary object to provide a novelcontrol means for optimizing the discharge currentbetween the electrodesof Y aprecipitator automaticallyoand maintaining` it at optimum valueseven thoughgthe/various factors aifecting the current may not allebe.-known' and may be continuously changing in relative valueor importance.i.

More particularly, Vit is'an object .of the invention toA provide anovel' apparatus for attaining' maximum parti-A clefrollectioneiciencygiin .asprecipitator whlcg .is `110,11

2,935,155 Patented May 3, 1960 fice dependent for operation on only oneof several inde-'J pendent variables all affecting such elliciency.`

Briey, these and further objects and advantages ofi the invention areattained by generating a signal that is a function of the average valueof the voltage acrossV the electrodes of the precipitator and passingthis signal to control apparatus for actuating a voltage regulatingmechanism in accordance with the rate of change of this signal.

substantially no possibility of arc-over occurring.

A better understanding of the principles underlying the apparatus of theinvention may be had by referring to the accompanying drawings, inwhich:

Fig. 1 is a block diagram of an electrical precipitatorf embodying themethod and apparatus of the present in-r vention; and

Fig. 2 is a graph of the average discharge current be tween theprecipitator electrodes of Fig. 1 plotted as a function of the averagevalue of the voltage across the electrodes.

Referring to Fig. 1, there is shown a precipitator comprising a pair ofopposing electrodes 10 and 11. These electrodes are arranged to collectparticles from a gas stream passing therebetween, and one of theelectrodes, 10

for example, may be in the form of a cylindricalmembery providing anextended surface on which particles collect, with the other electrode 11comprising a thin conducting member coaxially disposed within thecylinder and of a surface conguration to facilitate production of coronadischarge at the electrode. Usually, the outer plate or.

collecting electrode 10 is grounded, as shown.

These electrodes are energized from a high voltag Y source by a pair ofhigh voltage leads 12 and 13 from` oppositely disposed shoes 14 and 15respectivelyrof a mechanical rectiiier MR. The other two shoes of therectiiier are connected across the secondary 16 of a high voltagetransformer 17` having its primary 18 in turn con` nected to the outputof an auto transformer 19, whereby in place of the mechanical rectifierMR if desired. The elements described thus far are entirelyconventional.

Voltage from the source 20 is regulated by the auto-'1' transformer 19and stepped up to several thousand-lvolts by the power transformer 17.This high voltage is then rectified bythe rectifier MR and appliedacross the electrodes 1t) and 11 ofthe precipitator, the negative highvoltage terminal being connected to the center electrode Y 11 and theouter electrode 10 being connected to the grounded shoe of therectifier.

a discharge current from the electrode 10 towards the electrode 11. Theactual electron flow is from the centerA electrode 11 to the outerelectrode 10. This discharge current serves to charge particles in astream of gas passed Y' between the electrodes and causing the particlesto drift towards the relatively positive outer electrode 10. Control ofthis discharge current is effected by regulating the high voltage acrossthe electrodes by means of the auto A transformer 19.

As described earlier, a certain amount of sparking between theelectrodes is normal and is not serious. It is `found usually toexist'when the value of the discharge current approaches lthe `desiredoptimum value for maximum particle collection efliciency. Below thisoptimum value, the discharge current is caused largely if not enp'tirelyjby corona discharge atelectrodell, while Aabove The result ofthis operation is to maintain an". optimum average discharge currentbetween the electrodes resulting in maximum particle collection eciencywith The unidirectional elecf tric eld established between theelectrodes gives rise to t t-h'e optimum value sparking and eventuallyarc-over cause lthe major portion of the current ow. The optimum valueof this dis-charge current often changes continuously inasmuch as itdepends upon many independent variables such as atmospheric conditions,type of gas passing between the electrodes, the natu-re of the particlesbeing collected, etc. For a given installation, it can vary from hour tohour.

It has -been discovered that the average discharge current ilowingbetween the electrodes is a function of the average value, asdistinguished from the peak value, of the high voltage across theelectrodes. Fig. 2 shows a plot, in a qualitative way only, of thisfunction, as` found in practice in most precipitators ordinatesrepresenting average voltage across the precipitator electrodes and 11and the abscissae representing average discharge curn rent between theelectrodes. Plotting this graph may be accomplished by means ofk asuitable voltrneter, preferably of the electrostatic type so thatsubstantially no current is drawn, connected across the electrodes suchas Ishown at 21 in Fig. l. The average current reading between theelectrodes is then obtained from a meter circuit 2.2Y and milliameter 23of the type described and claimed-,in applicants -co-pending applicationSer. No. 422,440, led April l2, 1954 and entitled, Analyzer Circuit `forElectrical Precipitators, now abandoned. Prior to development of acircuitas disclosed in the above noted application, it was not .possibleto obtain sufliciently accurate readings of the average dischargecurrent to plot a curve such as shown in Fig. 2.

Referring :specifically to Fig. `2, it will be noted that thecurrent-voltage characteristic curve 24 rises as the voltage isincreased to a certain point and beyond that falls, doubling back onitself as the current increases. Thus, at certain voltages, andespecially within the range of normal operation, there are two possiblecurrent values. For example, at a voltage value indica-ted by thehorizontal line 25 two discharge current values I1 and I2 are possibleas noted at the points P-1 and P-2. On the other hand, at the voltagevalue indicated by the line 26, thereis but one optimum current value I3as indicated at the point P-3. The reason for the voltage drop after`passing the point P-3, with increasing discharge current, is thatsparking normally increases at a suiicient rate to prevent a furtherbuild up of the voltage.

Still further increase in the discharge current eventually results inarc-overs creating conditions represented by the dashed line 27. t

nCurve 24 is merely'typical of many curves actually plotted and shows ina general way the characteristic shape of the current voltage curve. It-generally resembles a 'second-degree or quadratic curve which, withinthev range` of normal operating voltages, has two current values for agiven applied voltage. Hence the average discharge current may be termeda double-value function of thel average applied voltage. 'Ih-ischar-acteristic is retained even though the exact shape of the curve maychange from one precipitator to another 6r from time to timefor the sameprecipitator. Because there are so many variables -actually enteringinto determination of the uiin-al curve, it may shift with reference tothe X and Y axes, but still retain its essential shape. Thus whileabsolu-te values may change, the shape of the curve giving `doublevalues for the average current as a function of average voltage isretained.

It will be seen from the above, that the optimum current values formaximum particle collection eiiciency lie close to the ideal point P-3,or within the shaded area 29, arc-over invariably occurring if thecurrent is increased too much beyond this point. The area of optimumoperation as represented by the shaded area may not remain in' oneposition but may vary with several inde- Y pendent factors, all in tu-rnvariable in themselves, including the sparking rate. The shaded area 29of `optimum Aoperation is, however, centered about the turning Cil pointor peak P-3 in the characteristic curve 24-that is, the point a-t whichthere is but one discharge current value yfor a given voltage.

The apparatus of the present invention is primarily concerned with asystem for maintaining the discharge current wit-hin a range of valuesas `defined by the shaded area 29. It will be immediately apparent thatat the desired optimum point P-3 of operation, the rate of change of thevoltage with respect to the current is zero. In accordance with theinvention, a control apparatus is employed for actuating theautotransforrner of Fig. l in accordance with this rate of change.

Mechanism lfor accomplishing this regulation is illus trated within thedashed box 30 of Fig. 1. As shown, `the apparatus includes high voltagelead 31 with an isolating current limiting resistance R connected to thecenter electrode li of the precipitator and terminating in a seriesresistance 32 grounded at its free end. A signal is generated acrossthis resistance which is proportional to the average voltage between theprecipitator electrodes. This signal is passed through a D.C. amplifiercircuit 33, of any suitable design. The output of the D.C. amplier 33 isserially connected through polarized relay coil 34 and storage`condenser 35 -to ground at 36. The relay and condenser are shunted by aresistance element 37. Flihe operation of this control portion of thecircuit Will be described shortly. Y

A reversible motor 38 is adapted to be connected to the power source 20through ya lead 39 and switch arm 4t) which is operated by relay coil 34to engage selectively terminal leads 411 and 42 of motor 38. Switch 40is normally biased to a neutral center position as shown in the drawing.Current passing throughthe polarized relay coil 34 in one directioncauses the switch -arm 4l) to contact the terminal lead 41 and energizeInotor 33 `to operate the same in one direction. On the other hand,current passing through the polarized relay coil 34-in the oppositedirection causes the switch arm 49 to contact the other terminal lead 42and energize the motor 38 to operate in the other or reverse direction.The center terminal of the motor is grounded as shown, and the outsideterminals 43 and 44 are connected to opposite ends of the motor fieldwindings. Operation of the motor in the'iirst direction moves the tap ofthe autotransformer to the right, as viewed in Fig. l to increase thevoltage fed to the primary 18 of transformer 17 and thus increase theaverage voltage across the electrodes of `the precipitator. Operation ofthe motor in the -reverse direction decreases this voltage.

- The direction of current flowing through the polarized relay coil 34is controlled by the signal developed across the resistance 32 and thetime-constant characteristics of the storage condenser 35 and returnpath resistance element 37.

4Referring once again to Fig. 2, assume that the precipitator is beingstarted up. As the voltage increases across the electrodes 10 and 11, asignal proportional to the changing value ofY this voltage developsacross the resistance 32 and is amplified in the D.C. amplilier 33. Asthis signal is increasing with time, the current through the relay runsfrom right to left and the storage condenser 35 commences charging.(hlrrent through the relay in the direction indicated by the solid linearrow energizes the relay coil to throw the switch arm 40 to the rightthus connecting power to the terminal lead 41 of the motor. The motorwill move the auto-tranformer tap to the right thereby continuing toincrease the voltage.

As the increasinggvoltage passes P-1, and approaches point P-3, its rateof change with respect to the discharge current approaches zero. The`value of the current ow from the D.C. amplifier 33 through thepolarized relay also approaches zero in view ofthe charging up of thestorage condenser 3S. Condenser 35 acts as an integrating circuit sinceit continuously accumulates charge as the voltage acrossresistance 32increases.- A back voltassures..

age s thus presented by the condenser tendingto buck the current flowVthrough the polarized relay until it reduces this current to zero. Atthis point, the condenser has a voltage charge proportional to thevoltage drop across the resistance 32. With no current owing through thepolarized relay, switch arm 40 returns to its neutral position therebyturning oli motor 38. The autotransformer tap is thus set automaticallyto provide an average high voltage across the precipitator electrodescorresponding to the point P-3.

If the average Value of the high voltage across the precipitatorelectrodes begins to decrease due to any one of several variable factorssuch as an increased sparking rate tending toward the point P-2, thecharge on the condenser 35 will be greater than the signal developed atthe output of the D.C. amplifier in view of the decreased voltage dropacross resistance 32. Current thus passes through the polarized relayfrom left to right, as indicated by the dotted arrow and throughresistance element 37 to ground. The relay 34 then connects the powersource by means of switch arm 40 to the motor terminal lead 43 andterminal 44 to start the motor in the reverse direction to lower thevoltage applied lby the autotransforrner. This initial decrease involtage immediately diminishes the discharge current owing between theelectrodes, resulting in an increasing of the actual voltage between theelectrodes. As will be seen from inspection of Fig. 2, the decreasingdischarge cur-y rent takes place along the curve from the point P-Ztowards the point P-S. This actual increase in voltage is reflectedacross the resistance 32 and sets up a bucking voltage against theaccumulated voltage on the condenserl 35. The reverse current llowthrough the polarized relay as indicated by the dotted arrow is therebystopped and the switch arm 40 will return to its neutral positionturning olf the motor 38.

It will be understood accordingly, that the condenser 35, resistance 37,polarized relay 34 and switch arm 40 act as a control circuit whichapplies a signal to the motor 3S proportional to the rate of change ofthe average voltage across the precipitator plates with respect to theaverage current between the plates. As can be seen from Fig. 2, thisrate of change is zero at the point P-3 and therefore, the setting ofthe autotransformer will not be changed once the point is reached.

It will also be appreciated that a change in the voltage is necessary tooperate the polarized relay. In other words, the control circuit tendsto cut itself off. In order then, to avoid the possibility of themechanism back tracking along the initial portion of the curve 24 ofFig. 2, or settling at some intermediate point below the point P-3,should no change in the voltage be present, a time delay circuit 45 isconnected to the neutral switch position of the arm 40 through lead 46,and to the terminal v 43 of the motor 38 through leads 47 and 48. Thistime delay circuit periodically applies power to terminal 43 to move theautotransformer tap to the right and thus increase the voltage,'apredetermined length of time after the switch arm 40 has returned to theneutral position. This period of time may be preset in accordance withthe type of installation, and may be any suitable length of time,preferably several minutes.

With this time delay arrangement, return of the switch arm 40 to theneutral position indicating a zero rate of change of the voltage isfollowed by power being applied to the motor after a predeterminedinterval of time. The resulting increase in voltage is reected by acurrent through the polarized relay to move the switch arm 4i? to themotor terminal 43 thereby cutting out the time delay circuit. But powerwill still be applied to the motor through switch arm 40 to continueincreasing the voltage until the point P3 is reached at which time thearm 40 will again return to neutral position. It is to be noted thatimmediately after application of a signal through the time delay 45, thetime delay circuit is cut out by movement of the switch arm 40 awayfrom.

the neutral position. Thus, the control circuit tends to always operateinside the shaded area 29 under the curve of Fig. 2.

In some instances, power interruptions may occur in thelow voltageportion of thesystem or in the power source 20. In order to prevent suchan interruption from adversely upsetting the balance of the controlcircuit, an under-current relay 49 is positioned to throw a switch 50 toapply power through leads 51 and 48 to the terminal 43 of the motor toincrease the voltage. When the trouble at the source is cured, theundercurrent relay will open switch 50 removing power from the motor andpermitting the control circuit to take over.

The above described method and apparatus for controlling electricalprecipitator thus insures than an optimum discharge current will alwaysresult, and the possibility of prolonged arc-overs is substantiallyeliminated. So long as the form of the characteristic curve remains thesame, the absolute values, which depend upon several variables, are notimportant. The control circuit is only concerned with maintaining thevoltage at a point where its rate of change with respect to thedischarge current is zero.

It is to be understood that different types of control circuits may beemployed. The particular apparatus illustrated in Fig. 1 is merelyillustrative, and other types responsive to the rate of change of thevoltage for controlling the discharge current may occur to those skilledin the art. Thus the scope and spirit of the present invention is not tobe thought of as limited to the particular apparatus disclosed.

I claim:

l. In an electrical precipitator, an apparatus for regulating the valueof the discharge current between high voltage and grounded electrodes ofsaid precipitator for maximum particle collection etciency comprising,in combination: signal generating means including a resistance connectedbetween the high voltage electrode of said precipitator and ground and adirect current amplitier connected across said resistance for amplifyingsignals appearing across said resistance, said generating meansproducing a signal that is a function of the average vol-tage across theelectrodes of said precipitator; regulating means in the low voltageportio-n of said precipitator for increasing or decreasing said averagevoltage; and control means responsive only to the rate of change of saidsignal for actuating said regulating means, said control meanscomprising a polarized relay coil and a storage condenser seriallyconnected across the output of said direct current amplifier; aresistance element shunting said serially connected relay coil andcondenser; and switch means operable in response to current ow in saidrelay coil for energizing said regulating means.

2. An apparatus according to claim l, in which said regulating meanscomprises a reversible motor and an autotransformer, said motor beingadapted to increase the output voltage of said autotransformer whenoperating in one direction, and decrease the output of saidautotransformer when operating in a reverse direction; said switch meansenergizing said motor in one direction when the current flow throughsaid polarized relay is in one direction, and reversing said motorwhenthe current liow through said relay is in a reverse direction.

3. An apparatus according to claim 2, including time delay means forapplying energy to said motor for increasing the voltage output of saidautotransformer a predetermined time after the `current ow through saidpolarized relay coil ceases, and means forrendering said time delayinoperative when current is owing through said polarized relay.

4. An apparatus according to claim 2, including an undercurrent relay inthe low voltage portion of said precipitator operatively coupled to saidmotor for increasing the voltage of said autotransformer when thecurrent to said autotransformer drps below a predetermined value.

References Cited in the le of this patent UNITED STATES PATENTS 8 Hall'Apr. 13, 1954 Klemperer Sept. 25, 1956 FOREIGN PATENTS Germany Jan. 14,1939 Great Britain May 17, 1940 Great Britain Mar. 17, 1954

